The invention relates to a digitally controlled circuit for automatically detecting an optimum A. C. recording bias signal level and storing that level in a memory device.
Magnetic recording/reproducing systems utilizing A. C. bias recording, for example, wideband audio tape recorders, require utilization of a precisely set optimum bias level for high quality recording. A recording technique known as overbiasing is commonly utilized to achieve the foregoing by obtaining a wide, flat frequency response characteristic and low harmonic distortion. For best quality of recording, tape manufacturers usually recommend a predetermined amount of overbias to be used with each type of magnetic tape. However, recording and playback characteristics differ with respect to various recording tapes and sometimes even when utilizing different batches of the same type of tape due to fluctuations in the magnetic characteristics of the particular transducer and tape utilized. Consequently, if optimum wideband recording characteristics are to be obtained, it is necessary to determine the exact amount of optimum recording bias level for each individual transducer and reel of tape, prior to recording.
In the past, an optimum bias signal level was known to be detected and manually set for each reel of tape prior to recording. The above method included manually increasing the amplitude of a bias signal having a predetermined frequency by adjusting a number of trimming potentiometers. The bias signal level was then mixed with an analog signal of known frequency and constant level and subsequently recorded. The resulting recorded signal was played back and a corresponding change in the output signal envelope obtained from the reproduce circuit was monitored. The maximum playback signal was first detected, then the recording bias level was further increased until the playback signal dropped to obtain a suggested overbias level and the resulting recording bias level was utilized for subsequent recording. Such manual method required operator skill, was time consuming, and, thus, costly.
There are also known prior art circuits as described, for example, in U.S. Pat. No. 4,011,585, which provide the above-described operations automatically by increasing the recording bias signal level. The corresponding reproduce signal is applied to both an envelope detector and a peak detector and the output of the peak detector is attenuated. When the obtained reproduce signal envelope level equals to the attenuated peak level, the operation is stopped and the corresponding recording bias level is detected by the circuit for use in subsequent recording.
It is a significant disadvantage that the known prior art circuits do not compensate for the distance between the recording and reproducing head, thus introducing substantial inaccuracy in the detected recording bias level. Such inaccuracy occurs due to a change in the recording bias level occurring during the time necessary for a point on tape to pass from the record head to the reproduce head. Besides, in the prior art devices the bias level increase and control is provided utilizing analog devices, thus, further contributing to inaccuracy of the detected signal level relative to the optimum recording bias level. Moreover, the prior art circuits do not provide for storing the detected bias level during power supply interruption or loss nor do they provide a range of selectable overbias levels for various types of tape for automatic recording bias detection.